A hybrid type working machine has a configuration in which a hydraulic pump is driven by an engine and an electric motor (a motor generator is generally used, and the “electric motor” mentioned in this specification includes the motor generator.) driven for power running by electric power stored in an electric storage device, and hydraulic actuators are driven by pressure oil ejected from the hydraulic pump so as to perform a required operation. Assume that when the hybrid type working machine is in operation, the amount of electricity stored in the electric storage device reaches such a lower limit value that the electric storage device cannot discharge electricity. In this case, it is necessary to suspend discharge from the electric storage device to thereby suspend the power running drive of the electric motor while changing a pump absorption horsepower maximum value to be not larger than the maximum rated torque of the engine.
However, when the mode is suddenly changed over to a mode in which the hydraulic pump is driven only by the engine in the stage where the amount of electricity stored in the electric storage device has reached the lower limit value, the engine cannot bear a load solely but may fall into engine stall. In addition, the pump absorption horsepower maximum value drops suddenly in the stage where the amount of electricity stored in the electric storage device has reached the lower limit value. Therefore, there is another problem that a feeling of strangeness on an operation may be given to an operator. Further, when the amount of electricity stored in the electric storage device reaches the lower limit value in the middle of a preset operation scheduled time of one day, the capability of the working machine deteriorates since then. Therefore, there arises another problem that a scheduled operation cannot be completed.
In order to solve such problems, there has been proposed a technique in which when the residual amount of electricity stored in the electric storage device decreases to be not larger than a first predetermined value set in advance, a pump absorption horsepower maximum value is reduced so as to limit a power-running torque value of an electric motor in accordance with the residual amount of electricity stored in an electric storage device at that time, and when the residual amount of electricity stored in the electric storage device decreases to be not larger than a second predetermined value set in advance, the power-running operation of the electric power is prohibited to further reduce the pump absorption horsepower maximum value down to such a value that the pump can be driven by the rated output of an engine (for example, see Patent Document 1). In this manner, while the hybrid type working machine is in operation, the pump absorption horsepower maximum value does not suddenly drop down to be not larger than the maximum rated torque of the engine, but the pump absorption horsepower maximum value is decreased stepwise in accordance with the residual amount of electricity stored in the electric storage device. Thus, engine stall can be avoided and a feeling of strangeness on an operation given to an operator can be relaxed.